Interaction between trypsin-like enzyme from Streptomyces erythraeus and chicken ovomucoid

Chicken ovomucoid (CO), an effective inhibitor of bovine trypsin, has a reactive site in each of three tandem domains. When CO was subjected to inhibition assay by the method of Green and Work, the second domain (CO Domain II) inhibited bovine trypsin but not TLE-Se, a trypsin-like enzyme from Streptomyces erythraeus, and the first domain (CO Domain I) inhibited neither bovine trypsin nor TLE-Se. However, when the interaction between CO and TLE-Se was analyzed by means of a Lineweaver-Burk plot, it was found that the ovomucoid exhibited competitive inhibition of the bacterial protease at pH 8.0 (Ki = 5.2 microM). In this case, the reactive-site peptide bonds of the first and second domains were specifically hydrolyzed. The isolated CO Domain I also exhibited competitive inhibition of TLE-Se (Ki = 3.1 microM), which specifically hydrolyzed its reactive-site peptide bond.     

Inhibition in vitro of lipogenic enzymes from bovine (Bos taurus) mammary tissue by methylmalonyl-coenzyme A and coenzyme A

Bovine mammary fatty acid synthetase was inhibited by approximately 50% by 40 microM methylmalonyl-CoA; this inhibition was competitive with respect to malonyl-CoA (apparent Ki = 11 microM). Similarly, 6.25 microM coenzyme A inhibited the synthetase by 35% and this inhibition was again competitive (apparent Ki = 1.7 microM). Apparent Km for malonyl-CoA was 29 microM. The short-chain dicarboxylic acids malonic, methylmalonic and ethylmalonic at high concentrations (160-320 microM) and ATP (5 mM) enhanced the synthetase activity by about 50% respectively; the activating effects of methylmalonic acid and ATP on the synthetase were additive. Methylmalonyl-CoA at 50 microM concentration inhibited the partially purified acetyl-CoA carboxylase uncompetitively by 10% and the propionyl-CoA carboxylase activity of the enzyme preparation competitively (apparent Ki = 21 microM) by 40%. Malonyl-CoA also inhibited the acetyl-CoA carboxylase activity competitively (apparent Ki = 7 microM) by 35% and the propionyl-CoA carboxylating activity of the preparation competitively (apparent Ki = 4 microM) by 82%. The possibility that methylmalonyl-CoA may be a causal factor in the aetiology of the low milk-fat syndrome in high yielding dairy cows is discussed.     

Steady-state kinetics of skeletal muscle myosin light chain kinase indicate a strong down regulation by products

The kinetic behaviour of myosin light chain kinase isolated from skeletal muscle was studied under steady-state conditions using highly purified phosphorylatable light chains 2 (LC2). Forward reaction, product inhibition, and reverse reaction data indicate a sequential mechanism which can be interpreted best by a rapid-equilibrium random bi-bi reaction model. The forward reaction parameters are KATP = 150 microM, KLC2 = 5.3 microM, and Ki LC2 = 7.6 microM. The enzyme forms a dead-end complex with ADP and light chain 2; Kd, ADP of this complex is 50 microM. The forward reaction is also strongly inhibited by the phosphorylated light chain 2, Ki, LC2P is 1.5 microM. An equilibrium constant Keq of about 70 can be calculated from the kinetic parameters which agrees with the directly measured value of about 60. The role of the two inhibitory mechanisms in the regulation of the enzyme and of the high energy of the light chain phosphate bond as deducible from Keq are discussed.     

Inhibition by melittin of phospholipid-sensitive and calmodulin-sensitive Ca2+-dependent protein kinases.

Effects of melittin, an amphipathic polypeptide, on various species of protein kinases were investigated. It was found that melittin inhibited the newly identified phospholipid-sensitive Ca2+-dependent protein kinase (from heart, brain, spleen and neutrophils) and the cardiac myosin light-chain kinase, a calmodulin-sensitive Ca2+-dependent enzyme. In contrast, melittin had little or no effect on either the holoenzymes of the cardiac cyclic AMP-dependent and cyclic GMP-dependent protein kinases or the catalytic subunit of the former. Kinetic analysis indicated that melittin inhibited phospholipid-sensitive Ca2+-dependent protein kinase non-competitively with respect to ATP (Ki = 1.3 microM); although exhibiting complex kinetics, its inhibition of the enzyme was overcome by phosphatidylserine (a phospholipid cofactor), but not by protein substrate (histone H1) or Ca2+. On the other hand, melittin inhibited myosin light-chain kinase non-competitively with respect to ATP (Ki = 1.4 microM) or Ca2+ (Ki = 1.9 microM), and competitively with respect to calmodulin (Ki = 0.08 microM); although exhibiting complex kinetics, its inhibition of the enzyme was reversed by myosin light chains (substrate protein). The present findings indicate the presence of functionally important hydrophobic or hydrophilic loci on the Ca2+-dependent protein kinases, but not on the cyclic nucleotide-dependent class of protein kinase, with which melittin can interact. Moreover, the kinetic data suggest that melittin inhibited myosin light-chain kinase by interacting with a site on the enzyme the same as, or proximal to, the calmodulin-binding site, thus interfering with the formation of active enzyme-calmodulin-Ca2+ complex.     

Inhibition of rat ovarian 3 beta-hydroxysteroid dehydrogenase (3 beta-HSD), 17 alpha-hydroxylase and 17,20 lyase by progestins and danazol

The site of action of synthetic progestins or danazol in the treatment of endometriosis is considered to be mainly the hypothalamo-pituitary level, but the direct action to the uterine endometrium and the ovary is also suggested. We investigated the effect of these synthetic steroids to rat ovarian steroidogenic enzymes. The effect of norethisterone, levonorgestrel, danazol, gestrinone, desogestrel and 3-keto-desogestrel was studied in vitro. The sources of the enzymes were prepared from ovaries of immature rats treated either with pregnant mare serum gonadotropin (PMS) and human chorionic gonadotropin (hCG) for 3 beta-hydroxy steroid dehydrogenase (3 beta-HSD), or with PMS for 17 alpha-hydroxylase and 17,20 lyase. The substrates used were pregnenolone (P5) for 3 beta-HSD, progesterone (P4) for 17 alpha-hydroxylase, and 17 alpha-hydroxy-progesterone (17 alpha-OH-P4) for 17,20 lyase. The substrates were incubated with the enzyme sources and coenzymes, and the products formed were measured. All the steroids inhibited 3 beta-HSD, and the inhibition by gestrinone (Ki = 3.0 microM) and 3-keto-desogestrel (17.5 microM) was particularly marked. Only desogestrel (Ki = 30.3 microM) and danazol (168 microM) inhibited 17 alpha-hydroxylase. All the steroids inhibited 17,20 lyase, and the inhibition by desogestrel (Ki = 0.70 microM), danazol (0.80 microM), and gestrinone (30 microM) was particularly marked.     

Allosteric regulation of purine nucleoside phosphorylase.

Purine nucleoside phosphorylase (EC 2.4.2.1) from bovine spleen is allosterically regulated. With the substrate inosine the enzyme displayed complex kinetics: positive cooperativity vs inosine when this substrate was close to physiological concentrations, negative cooperativity at inosine concentrations greater than 60 microM, and substrate inhibition at inosine greater than 1 mM. No cooperativity was observed with the alternative substrate, guanosine. The activity of purine nucleoside phosphorylase toward the substrate inosine was sensitive to the presence of reducing thiols; oxidation caused a loss of cooperativity toward inosine, as well as a 10-fold decreased affinity for inosine. The enzyme also displayed negative cooperativity toward phosphate at physiological concentrations of Pi, but oxidation had no effect on either the affinity or cooperativity toward phosphate. The importance of reduced cysteines on the enzyme is thus specific for binding of the nucleoside substrate. The enzyme was modestly inhibited by the pyrimidine nucleotides CTP (Ki = 118 microM) and UTP (Ki = 164 microM), but showed greater sensitivity to 5-phosphoribosyl-1-pyrophosphate (Ki = 5.2 microM).     

Inhibition by gossypol of phospholipid-sensitive Ca2+-dependent protein kinase from pig testis

Gossypol, a polyphenolic binaphthalene-dialdehyde extracted from cotton plants which possesses male antifertility action in mammals, is a potent inhibitor of phospholipid-sensitive Ca2+-dependent protein kinase from pig testis. Gossypol inhibited Ca2+-dependent activity of the enzyme without affecting its basal activity. The IC50 value (concentration causing 50% inhibition) was 31 microM when lysine-rich histone was used as substrate. Kinetic analysis indicated that the compound inhibited the enzyme non-competitively with respect to ATP (Ki = 31 microM) or lysine-rich histone (Ki = 30 microM), and competitively with respect to phosphatidylserine (Ki = 2.1 microM). With Ca2+, irrespective of the presence or absence of 1,3-diolein, the compound lowered Vmax and increased the apparent Ka for Ca2+. The compound also inhibited phosphorylation by the enzyme of high-mobility-group 1 protein (one of the endogenous substrates in the testis for the enzyme located in nucleosome), with an IC50 value of 88 microM. These results suggested that a phospholipid-sensitive Ca2+-dependent protein phosphorylation system in the testis is involved in the regulation of spermatogenesis.     

Inactivation of delta 5-3-oxo steroid isomerase with active-site-directed acetylenic steroids.

Several steroid analogues containing conjugated acetylenic ketone groups as part of a seco-ring structure or as substituents on the intact steroid system are irreversible inhibitors of delta 5-3-oxo steroid isomerase (EC 5.3.3.1) from Pseudomonas testosteroni. Thus 10 beta-(1-oxoprop-2-ynyl)oestr-4-ene-3,17-dione (I), 5,10-seco-oestr-4-yne-3,10,17-trione (II), 17 beta-hydroxy-5,10-seco-oestr-4-yne-3,10-dione (III) and 17 beta-(1-oxoprop-2-ynyl)androst-4-en-3-one (IV) irreversibly inactivate isomerase in a time-dependent manner. In all cases saturation kinetics are observed. Protection against inactivation is afforded by the powerful competitive inhibitor 19-nortestosterone. The inhibition constants (Ki) for 19-nortestosterone obtained from such experiments are in good agreement with those determined from conventional competitive-inhibition studies of enzyme activity. These compounds thus appear to be active-site directed. In every case the inactivated enzyme could be dialysed without return of activity, indicating that a stable covalent bond probably had formed between the steroid and enzyme. Compound (I) is a very potent inhibitor of isomerase [Ki = 66.0 microM and k+2 = 12.5 x 10(-3) s-1 (where Ki is the dissociation constant of the reversible enzyme-inhibitor complex and k+2 is the rate constant for the inactivation reaction of the enzyme-inhibitor complex)] giving half-lives of inactivation of 30-45 s at saturation. It is argued that the basic-amino-acid residue that abstracts the intramolecularly transferred 4 beta-proton in the reaction mechanism could form a Michael-addition product with compound (I). In contrast, although compound (IV) has a lower inhibition constant (Ki = 14.5 microM), it is a relatively poor alkylating agent (k+2 = 0.13 x 10(-3) s-1). If the conjugated acetylenic ketone groups are replaced by alpha-hydroxyacetylene groups, the resultant analogues of steroids (I)-(IV) are reversible competitive inhibitors with Ki values in the range 27-350 microM. The enzyme binds steroids in the C19 series with functionalized acetylenic substituents at C-17 in preference to steroids in the C18 series bearing similar groups in the ring structure or as C-10 substituents. In the 5,10-seco-steroid series the presence of hydroxy groups at both C-3 and C-17 is deleterious to binding by the enzyme.     

Inhibitory effect of long chain fatty acyl CoAs on RNA polymerase from Escherichia coli

RNA polymerase from Escherichia coli was inhibited by long chain fatty acyl CoAs, such as myristoyl CoA (Ki = 17.2 microM), palmitoyl CoA (Ki = 8.9 microM), oleoyl CoA (Ki = 5.5 microM), and stearoyl CoA (Ki = 0.94 microM). The inhibition by these CoA thioesters was non-competitive against nucleoside triphosphates. Short chain fatty acyl CoAs, such as acetyl CoA, propionyl CoA, acetoacetyl CoA, butyryl CoA, and decanoyl CoA, failed to inhibit RNA polymerase. CoA, Na-myristate, Na-palmitate, Na-oleate, Na-stearate, palmitoyl carnitine, and carnitine did not inhibit the enzyme. The inhibition of RNA polymerase by long chain fatty acyl CoAs was competitive against template DNA.     

Kinetic characterization of the rotenone-insensitive internal NADH: ubiquinone oxidoreductase of mitochondria from Saccharomyces cerevisiae

Saccharomyces cerevisiae mitochondria contain an NADH:Q6 oxidoreductase (internal NADH dehydrogenase) encoded by NDI1 gene in chromosome XIII. This enzyme catalyzes the transfer of electrons from NADH to ubiquinone without the translocation of protons across the membrane. From a structural point of view, the mature enzyme has a single subunit of 53 kDa with FAD as the only prosthetic group. Due to the fact that S. cerevisiae cells lack complex I, the expression of this protein is essential for cell growth under respiratory conditions. The results reported in this work show that the internal NADH dehydrogenase follows a ping-pong mechanism, with a Km for NADH of 9.4 microM and a Km for oxidized 2,6-dichorophenolindophenol (DCPIP) of 6.2 microM. NAD+, one of the products of the reaction, did not inhibit the enzyme while the other product, reduced DCPIP, inhibited the enzyme with a Ki of 11.5 microM. Two dead-end inhibitors, AMP and flavone, were used to further characterize the kinetic mechanism of the enzyme. AMP was a linear competitive inhibitor of NADH (Ki = 5.5 mM) and a linear uncompetitive inhibitor of oxidized DCPIP (Ki = 11.5 mM), in agreement with the ping-pong mechanism. On the other hand, flavone was a partial inhibitor displaying a hyperbolic uncompetitive inhibition regarding NADH, and a hyperbolic noncompetitive inhibition with respect to oxidized DCPIP. The apparent intercept inhibition constant (Kii = 5.4 microM) and the slope inhibition constant (Kis = 7.1 microM) were obtained by non linear regression analysis. The results indicate that the ternary complex F-DCPIPox-flavone catalyzes the reduction of DCPIP, although with lower efficiency. The effect of pH on Vmax was studied. The Vmax profile shows two groups with pKa values of 5.3 and 7.2 involved in the catalytic process.     

Prostaglandin-E2 9-ketoreductase from human uterine decidua vera

Prostaglandin-E2 9-ketoreductase, the enzyme which catalyzes the reaction from prostaglandin E2 (PGE2) to prostaglandin F2 alpha (PGF2 alpha), has been purified 232-fold from human uterine decidua vera. The molecular mass of the enzyme, as estimated by fast protein liquid chromatography, was 29 kDa. Sodium dodecyl sulfate disc gel electrophoresis of the denatured enzyme revealed a molecular mass of 31 kDa. These data suggest that the enzyme consists of a single polypeptide chain. The rate equation of the enzyme reaction for two substrates was used for the determination of five kinetic constants. The equilibrium constant with respect to PGE2 was 83 microM, the Michaelis constant, Km, for PGE2 was 93 microM. For NADPH, the equilibrium constant was 1.0 microM and Km was 1.6 microM. The maximal velocity for the forward reaction was V1 = 217 pmol/min. The inhibition constants for the analgesic agents indomethacin and fentiazac were Ki = 850 microM and Ki = 450 microM and for the steroid progesterone Ki = 1.5 mM, respectively. Prostaglandin-E2 9-ketoreductase might be responsible for the control of the PGE2/PGF2 alpha ratio in human decidua vera. The enzyme, therefore, might be an important factor in the cascade of events leading to uterine contractions and parturition.     

Purification and characterization of leukotriene A4 epoxide hydrolase from dog lung

Leukotriene A4 epoxide hydrolase from dog lung, a soluble enzyme catalyzing the hydrolysis of leukotriene A4 (LTA4) to leukotriene B4 (LTB4) was partially purified by anion exchange HPLC. The enzymatic reaction obeys Michaelis- Menten kinetics. The apparent Km ranged between 15 and 25 microM and the enzyme exhibited an optimum activity at pH 7.8. An improved assay for the epoxide hydrolase has been developed using bovine serum albumin and EDTA to increase the conversion of LTA4 to LTB4. This method was used to produce 700 mg of LTB4 from LTA4 methyl ester. The partial by purified enzyme was found to be uncompetitively inhibited by divalent cations. Ca+2, Mn+2, Fe+2, Zn+2 and Cu+2 were found to have inhibitor constants (Ki) of 89 mM, 3.4 mM, 1.1 mM, 0.57 mM, and 28 microM respectively Eicosapentaenoic acid was shown to be a competitive inhibitor of this enzyme with a Ki of 200 microM. From these inhibition studies, it can be theorized that the epoxide hydrolase has at least one hydrophobic and one hydrophilic binding site.     

Purification and characterization of bile salt sulfotransferase from human liver

Bile salt sulfotransferase, the enzyme responsible for the formation of bile salt sulfate esters, was purified extensively from normal human liver. The purification procedure included DEAE-Sephadex chromatography, taurocholate-agarose affinity chromatography, and preparative isoelectrofocusing. The final preparation had a specific activity of 18 nmol min-1 mg protein-1, representing a 760-fold purification from the cytosol fraction with a overall yield of 15%. The human enzyme has a Mr of 67,000 and a pI of 5.2. DEAE-Sephadex chromatography of the cytosol fraction revealed only a single species of activity. The limiting Km for the sulfuryl donor, 3'-phosphoadenosine-5'-phosphosulfate (PAPS), is 0.7 microM. The limiting Km for the sulfuryl acceptor, glycolithocholate (GLC), is 2 microM. Reciprocal plots were intersecting. Product inhibition studies established that adenosine 3',5'-diphosphate (PAP) was competitive with PAPS (Ki = 0.2 microM) and noncompetitive with respect to GLC. GLC sulfate was competitive with GLC (Ki = 2.2 microM) and noncompetitive with respect to PAPS. Also, 3-ketolithocholate, a dead-end inhibitor, was competitive with GLC (Ki = 0.6 microM) and noncompetitive with respect to PAPS. Iso-PAP (the 2' isomer of PAP) was competitive with PAPS (Ki = 0.3 microM) and noncompetitive with GLC. The cumulative results of the steady-state kinetics experiments point to a random mechanism for the binding of substrates and release of products. The purified enzyme displays no activity toward estrone, testosterone, or phenol. Among the reactive substrates tested, the Vmax/Km values are in the order GLC greater than 3-beta OH-5-cholenic acid greater than glycochenodeoxycholate greater than glycocholate. p-Chloromercuribenzoate inactivated the enzyme. Either PAPS or GLC protected against inactivation, suggesting the presence of a sulfhydryl group at the active site.     

Inhibition of human leukocyte 3-hydroxy-3-methylglutaryl coenzyme A reductase activity by ascorbic acid. An effect mediated by the free radical monodehydroascorbate

3-Hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase activity in microsomes isolated from cultured lymphoid (IM-9) cells or freshly isolated human leukocytes was markedly decreased by either ascorbic acid or its oxidized derivative, dehydroascorbate. Inhibition of IM-9 leukocyte HMG-CoA reductase activity was log linear between 0.01 and 10 mM ascorbic acid (25 and 81% inhibition, respectively) and 0.1 and 10 mM dehydroascorbate (5 and 75% inhibition, respectively). Inhibition was noncompetitive with respect to HMG-CoA (Km = 10.2 microM (RS); ascorbic acid, Ki = 6.4 mM; dehydroascorbate, Ki = 15 mM) and competitive with respect to NADPH (Km = 16.3 microM; acetic acid, Ki = 6.3 mM; dehydroascorbate, Ki = 3.1 mM). Ascorbic acid and dehydroascorbate are interconverted through the free radical intermediate monodehydroascorbate. Reducing agents are required to convert dehydroascorbate to monodehydroascorbate, but prevent formation of the free radical from ascorbate. In microsomes from IM-9 cells, the reducing agent, dithiothreitol, abolished HMG-CoA reductase inhibition by ascorbate but enhanced inhibition by dehydroascorbate. In addition, the concentration of monodehydroascorbate present in ascorbate solutions was directly proportional to the degree of HMG-CoA reductase inhibition by 1.0 mM ascorbate. Fifty per cent inhibition of enzyme activity occurred at a monodehydroascorbate concentration of 14 microM. These data indicate that monodehydroascorbate mediates inhibition of HMG-CoA reductase by both ascorbate and dehydroascorbate. This effect does not appear to be due to free radical-induced membrane lipid modification, however, since both ascorbate and dehydroascorbate inhibited the protease-solubilized, partially purified human liver enzyme. Since inhibition of HMG-CoA reductase occurs at physiological concentrations of ascorbic acid in the human leukocyte (0.2-1.72 mM), this vitamin may be important in the regulation of endogenous cholesterol synthesis in man.     

The K+-translocating Kdp-ATPase from Escherichia coli. Purification, enzymatic properties and production of complex- and subunit-specific antisera

The Kdp system from Escherichia coli is a derepressible high-affinity K+-uptake ATPase. Its membrane-bound ATPase activity was approximately 50 mumol g-1 min-1. The Kdp-ATPase complex was purified from everted vesicles by solubilization with the nonionic detergent Aminoxid WS 35 followed by DEAE-Sepharose CL-6B chromatography at pH 7.5 and pH 6.4 and gel filtration on Fractogel TSK HW-65. The overall yield of activity was 6.5% and the purity at least 90%. The isolated KdpABC complex had a high affinity for its substrates K+ (Km app. = 10 microM) and Mg2+-ATP (Km = 80 microM) and a narrow substrate specificity. The ATPase activity was inhibited by vanadate (Ki = 1.5 microM), fluorescein isothiocyanate (Ki = 3.5 microM), N,N'-dicyclohexylcarbodiimide (Ki = 60 microM) and N-ethylmaleimide (Ki = 0.1 mM). The purification protocol was likewise applicable to the isolation of a KdpA mutant ATPase which in contrast to the wild-type enzyme exhibited an increased Km value for K+ of 6 mM and a 10-fold lowered sensitivity for vanadate. Starting from the purified Kdp complex the single subunits were obtained by gel filtration on Bio-Gel P-100 in the presence of SDS. Both the native Kdp-ATPase and the SDS-denatured polypeptides were used to raise polyclonal antibodies. The specificity of the antisera was established by immunoblot analysis. In functional inhibition studies the anti-KdpABC and anti-KdpB sera impaired ATPase activity in the membrane-bound as well as in the purified state of the enzyme. In contrast, the anti-KdpC serum did not inhibit enzyme activity.     

Kinetic characterization of the pyruvate and oxoglutarate dehydrogenase complexes from human heart

The Michaelis constant values for the highly purified pyruvate dehydrogenase complex (PDC) from human heart are 25, 13 and 50 microM for pyruvate, CoA and NAD, respectively. Acetyl-CoA produces a competitive inhibition of PDC (Ki = 35 microM) with respect to CoA, whereas NADH produces the same type of inhibition with respect to NAD (Ki = 36 microM). The oxoglutarate dehydrogenase complex (OGDC) from human heart has active sites with two different affinities for 2-oxoglutarate ([S]0.5 of 30 and 120 microM). ADP (1 mM) decreases the [S]0.5 values by a half. The inhibition of OGDC (Ki = 81 microM) by succinyl-CoA is of a competitive type with respect to CoA (Km = 2.5 microM), whereas that of NADH (Ki = 25 microM) is of a mixed type with respect to NAD (Km = 170 microM).     

Inhibition of bovine heart NAD-specific isocitrate dehydrogenase by reduced pyridine nucleotides: modulation of inhibition by ADP, NAD+, Ca2+, citrate, and isocitrate

The activity of NAD-dependent isocitrate dehydrogenase from bovine heart was inhibited by NADH (apparent Ki about 4.3 microM) and NADPH (Ki about 9.8 microM) at subsaturating substrate concentrations at pH 7.4. The inhibition by NADH or NADPH was reversed competitively by magnesium isocitrate in the presence of ADP, but not without ADP. Reversal of inhibition by NADH or NADPH with respect to NAD+ was competitive or of the linear mixed type depending on whether ADP was absent or present. ADP3- (0.2 mM) increased the Ki(app) for NADPH from 9.8 to 27.1 microM; further addition of Ca2+ (0.2 mM) raised the Ki(app) to 127 microM. For the modification of NADPH inhibition by ADP, S0.5 for Ca2+ was approximately 48 microM. This compares to the Km for Ca2+ of 0.3-1 microM for the activation of the enzyme without NADPH [Denton, R. M., Richards, D. A., & Chin, J. G. (1978) Biochem. J. 176, 899-906; Aogaichi, T., Evans, J., Gabriel, J., & Plaut, G. W. E. (1980) Arch. Biochem. Biophys. 204, 350-360]. ADP did not affect the Ki for NADH. Magnesium citrate, which was about 100-fold more effective as a positive modifier of the enzyme with ADP than without ADP [Gabriel, J. L., & Plaut, G. W. E. (1983) Fed. Proc., Fed. Am. Soc. Exp. Biol. 42, 2082], reversed competitively the inhibition by NADPH in the presence of ADP, but not without ADP. Magnesium citrate did not reverse NADH inhibition.(ABSTRACT TRUNCATED AT 250 WORDS)     

Competitive inhibitors of rabbit hepatic microsomal steroid 12 alpha-hydroxylase

The sterols 7 alpha-hydroxycholest-4-en-3-one (I) and 5 alpha-cholestane-3 alpha,7 alpha-diol (II) are competitive inhibitors for rabbit hepatic microsomal preparations of steroid 12 alpha-hydroxylase with apparent Ki values of 56 and 93 microM, respectively. To ascertain the optimum structure for a substrate with maximal enzymic activity, nine sterols or steroidal acids containing the 7 alpha-hydroxy-4-en-3-one or 3 alpha,7 alpha-dihydroxy-5 alpha configuration were prepared and studied as inhibitors with enzyme preparations in the presence of NADPH, oxygen and appropriate cofactors. Although each of these compounds exhibited competitive inhibition, the best inhibitor for sterol (I) was 7 alpha,25-dihydroxycholest-4-en-3-one (IV) (Ki 36 microM). Steroidal acids (3-oxo-7 alpha-hydroxychol-4-enoic acid and 3-oxo-7 alpha-hydroxy-4-cholene-24-carboxylic acid) were poor inhibitors (Ki 1080 and 654 microM, respectively). For sterol (II) the best inhibitors were sterol (IV) (Ki 35 microM) and 5 alpha-cholestane-3 alpha,7 alpha,25-triol (VIII) (Ki 45 microM). The 12 alpha-hydroxylated products of sterols (I) and (IV) were less tightly bound to the enzyme (Ki 88 and 98 microM, respectively) in the presence of sterol (II). Allochenodeoxycholic acid (Ki 495 microM) was not a good inhibitor for sterol (II). 12 alpha-Hydroxylated products of sterols (IV) and (VIII) were isolated from larger scale incubations, separated by HPLC and identified by mass spectrometry.     

Pseudomonas aeruginosa possesses two novel regulatory isozymes of 3-deoxy-D-arabino-heptulosonate 7-phosphate synthase

In Pseudomonas aeruginosa the initial enzyme of aromatic amino acid biosynthesis, 3-deoxy-D-arabinoheptulosonate 7-phosphate (DAHP) synthase, has been known to be subject to feedback inhibition by a metabolite in each of the three major pathway branchlets. Thus, an apparent balanced multieffector control is mediated by L-tyrosine, by L-tryptophan, and phenylpyruvate. We have now resolved DAHP synthase into two distinctive regulatory isozymes, herein denoted DAHP synthase-tyr (Mr = 137,000) and DAHP synthase-trp (Mr = 175,000). DAHP synthase-tyr comprises greater than 90% of the total activity. L-Tyrosine was found to be a potent effector, inhibiting competitively with respect to both phosphoenolpyruvate (Ki = 23 microM) and erythrose 4-phosphate (Ki = 23 microM). Phenylpyruvate was a less effective competitive inhibitor: phosphoenolpyruvate (Ki = 2.55 mM) and erythrose 4-phosphate (Ki = 1.35 mM). DAHP synthase-trp was found to be inhibited noncompetitively by L-tryptophan with respect to phosphoenolpyruvate (Ki = 40 microM) and competitively with respect to erythrose 4-phosphate (Ki = 5 microM). Chorismate was a relatively weak competitive inhibitor: phosphoenolpyruvate (Ki = 1.35 mM) and erythrose 4-phosphate (Ki = 2.25 mM). Thus, each isozyme is strongly inhibited by an amino acid end product and weakly inhibited by an intermediary metabolite.     

Carbonyl sulfide inhibition of CO dehydrogenase from Rhodospirillum rubrum

Carbonyl sulfide (COS) has been investigated as a rapid-equilibrium inhibitor of CO oxidation by the CO dehydrogenase purified from Rhodospirillum rubrum. The kinetic evidence suggests that the inhibition by COS is largely competitive versus CO (Ki = 2.3 microM) and uncompetitive versus methylviologen as electron acceptor (Ki = 15.8 microM). The data are compatible with a ping-pong mechanism for CO oxidation and COS inhibition. Unlike the substrate CO, COS does not reduce the iron-sulfur centers of dye-oxidized CO dehydrogenase and thus is not an alternative substrate for the enzyme. However, like CO, COS is capable of protecting CO dehydrogenase from slow-binding inhibition by cyanide. A true binding constant (KD) of 2.2 microM for COS has been derived on the basis of the saturable nature of COS protection against cyanide inhibition. The ability of CO, CO2, COS, and related CO/CO2 analogues to reverse cyanide inhibition of CO dehydrogenase is also demonstrated. The kinetic results are interpreted in terms of two binding sites for CO on CO dehydrogenase from R. rubrum.